The use of power converters in grid-connected applications has become a popular subject in the last few years. In distributed generation systems, these power converters act as active interfaces, and can be composed of a VSI connected to the grid by means of a simple series L filter. However, to avoid injection of switching harmonics, this solution is limited to high switching frequencies. In contrast, it has been observed that the use of inverters with LCL filters can achieve reduced levels of harmonic distortion, even at low switching frequencies and with a smaller inductance. Therefore, they have become a more compact and economically more convenient solution. This makes the inverters with LCL filters good candidates for higher power applications where the switching frequency is limited. The applicability of these systems includes static VAR compensators, uninterruptible power systems, power flow compensators, and distributed generation system interfaces (photovoltaic, wind power, micro turbine, fuel cell, etc.), among others. In distributed generation applications, the primary energy source is connected to the grid by means of a power converter, such as a voltage source inverter (VSI), and a filter. Originally, an L filter was used, but recently LCL filter-based converters are becoming more popular due to their improved characteristics that allow them to comply with the increasingly more restrictive standards. These systems, however, present more complex dynamics than L filter-based converters. Besides the increased complexity of the LCL filter, a resonance arises, which compromises the stability of the system and makes the system more susceptible to grid disturbances. Therefore, more sophisticated controller methods are desired to guarantee stability with enhanced disturbance rejection capability. In particular, the control schemes should be able to alleviate the harmonics distortion which has become an almost mandatory feature for the controller design. Simultaneously, the implementation of such controllers should be kept as simple as possible and minimize or dispense with the requirement of additional sensors to keep the implementation cost comparable to known L filter-based converters.
The present disclosure makes reference to thirteen (13) documents identified in the References section below. Each of these documents is incorporated by reference in their entireties. For conciseness, the documents are identified herein with respect to the numeral assigned thereto in the References section below. In document [1], the authors present a comparison between PV inverters with an L filter and with an LCL filter. They show that with both schemes the low frequency harmonics attenuation is more and less the same. However, in the LCL filter, the switching harmonics are better attenuated. For instance, the LCL filter makes it possible to comply with EMC standards with relatively low switching frequencies. The authors observe that in LCL filters control becomes more expensive and complex. The proposed controller requires measurement of all variables.
In document [2], the authors propose to use similar controllers for the LCL filter as in the L filter inverter. They consider that at low frequencies the response between a single L filter and an LCL filter is similar. The authors use PI controllers for both the current and the DC link voltage controllers. Moreover, they propose to add a passive resistor in parallel with the outer inductor to somehow improve stability at the cost of dissipation losses. The inverter is controlled to emulate a resistor so that the current will be a scaled version of the grid voltage. However, if the grid voltage is distorted, then the current will also be distorted. The voltage outer loop is used to produce the current reference, and thus its bandwidth is made very small to avoid further distortion reinjection.
In document [3], the authors present a proportional-plus-resonant (P+R) controller. They measure the grid-side current and the capacitor current. This is for a three-phase system. They study the effect of the harmonic distortion in the grid voltage, however, only to propose to tune the controller to somehow alleviate this issue.
In document [4], the authors propose a controller which requires that only the current on the inverter side and the grid voltage are measured. The scheme is based on a P+R controller for the stability and tracking of the fundamental, and includes a bank of harmonic oscillators for the compensation of harmonic distortion (HC). However, the proposed scheme controls the inverter-side current rather than the grid current, and thus it may experience some inaccuracies on the delivered output current. The authors also propose to include the delay due to sampling, which apparently improves stability. Then, in documents [5] to [6], they use basically the same controller, except that this time they measure the current on the grid side. However, direct application of this controller to the grid current may entail stability issues as there is missing damping that cannot be injected with a simple P+R controller.
It is clear that with the LCL filter, a resonance is introduced, and thus efforts should be made to somehow damp this resonance and preserve stability. This process of damping the resonance is referred to as active damping injection. Different approaches for active damping injection in LCL filters have been proposed so far in documents [7] to [10].
In document [7], the authors propose the use of a lead-lag compensator loop on the capacitor voltage to actively damp the resonance of the LCL filter. Other works use the feedback of the capacitor current (document [10]), and some others require the feedback of all state variables of the LCL filter (document [9]). However, the use of additional measurements increases the cost as more sensors are required. The introduction of complex poles and complex zeros, as well as the introduction of a notch filter around the resonance, are also other techniques reported in document [8]. However, as noticed by the authors, the tuning of such schemes is sensitive to system parameters, and the active damping injection could become ineffective in case of a weak grid.
In document [8], the authors propose the use of a P+R as the current controller. They realize that in the low frequencies range, the stability conditions are imposed mainly by the P+R controller. However, at high frequencies, stability is more related to the damping of the LCL filter itself, with very small influence from the P+R controller. This motivates the use of mechanisms to insert extra damping so stability can be guaranteed. The authors propose to inject active damping by inserting two zeros around the resonance frequency. Moreover, in the case that the converter current is the measured variable, they propose to include two active damping poles to somehow compensate the resonant zeros of the system. They also study another method that consists of inserting a notch filter around the system resonance. The authors show that in case of a weak grid, the active damping injection could be ineffective, and thus provisions must be taken to properly tune the controller.
In document [9], the authors propose a controller which is a cascade interconnection of PI and deadbeat controllers. The PI is used as an outer loop to control the grid current, delivering the reference for the inverter-side current, which is then stabilized by the deadbeat controller. The controller requires the measurements of all variables in the LCL filter, and thus their feedback on the controller represents the stabilization mechanism. The effect of the harmonic distortion in the grid voltage is studied, but no harmonic mechanism is included to overcome this issue. Rather, this compensation is left to the frequency characteristics of the cascade controller.
In document [10], the authors propose modifications to the conventional DPC control to consider the LCL filter. They propose to alleviate the resonance issue by injecting active damping. For this purpose, the capacitor current is measured in addition to the converter-side current. A harmonic compensation scheme is also presented, which is based on the synchronous reference frame representations of signals.